In recent years there has been a trend towards avoiding the large amount of wastage which is normal with single use cardboard shipping containers by using containers of more durable material which can be disassembled after use, collapsed to a relatively small space, and returned to the shipper for re-use. One kind of durable material which has been used for this purpose is a double walled, fluted plastic material having an outer and an inner sheet which are joined together by a series of parallel webs integral with the outer and inner sheets; such material can be formed by extrusion. This material can be much stronger than cardboard, has energy absorbing properties, and is also waterproof. Suitable plastic is polypropylene. The use of such material for returnable shipping containers is shown for example in the following U.S. Pat. Nos.:
4,948,039, issued Aug. 14, 1990 to Amatangelo; and PA1 5,292,060, issued Mar. 8, 1994 to Focke et at. PA1 a molded base having a container bottom plate surrounded by an upstanding flange; PA1 a molded top closure having a container top plate surrounded by a depending flange; and PA1 a sleeve part suitable for providing sidewalls surrounding a space between the base and the top closure and having upper and lower margins suitable for being received within in said flanges, said sleeve part also including hinge sections connecting said sidewalls.
In both of these prior patents, the container is entirely formed from the double walled plastic material, and special provision is made to provide hinge joints between panels of the normally rigid material. However, in these constructions, the bottom of the container may be undesirably weak for heavy materials. Also, in both patents, at least some of the sidewalls have the webs extending horizontally rather than vertically; vertical webs would be preferable from the strength point of view, for resisting vertical loads, especially since it is usual to stack containers one on top of another.